kb1408

A disk of radius 2.4 cm carries a uniform surface charge density of 3.1 μ C/m2. Using reasonable approximations, find the electric field on the axis at the following distances.

I have used the equation E=(Q/ε₀)(1/(4*pi*r²))
I also tried the equation E=(Q/2(ε₀))(1-(z/(√(z²)+(r²)))

Thanks in advance for the help. Both equations have not led me to the correct answer.

*note, there is not a figure provided for this question*